The present invention relates to an apparatus and method for controlling a driving force derived from an internal combustion engine to driven wheels of an automotive vehicle, especially related to the vehicular driving force controlling apparatus and method in which an intake air quantity to the engine is adjusted and a fuel supply to the engine is cut off or reduced so as to control an output of the engine, thus enabling the control of the driving force over each driven road wheel.
When a vehicular driver depresses deeply an accelerator pedal of the vehicle to accelerate the vehicle, such as a rear wheel driven road wheel vehicle, two driven road wheels slip largely than required so that a sufficient acceleration characteristic and a sufficient steering stability cannot be achieved in a case where the vehicle runs on a small (a low) road frictional coefficient which is smaller or lower than the driver's image. The road frictional coefficient is herein after also called .mu.. To avoid such an inconvenience as described above, various types of the driving force controlling apparatuses have been proposed in which a control of a driving torque transmitted to the driven wheels is carried out so that the driving force acted upon between the driven road wheels and the running road surface is in accordance with the road frictional coefficient .mu. so as to suppress the slips on the driven road wheels and so as to assure the acceleration characteristic: of the vehicle and running stability (steering stability) thereof.
There are a variety of objects to be controlled by the above-described driving force controlling apparatus and controlled variables thereof.
One of the driving force controlling means (or apparatuses) includes such means (or apparatuses) arranged for imparting a braking force to the driven road wheels on which a slippage occurs, specifically, for forcefully activating a wheel cylinder (s) of a braking system to control the driving force (hereinafter, also referred to as a braking force control type driving force controlling means (or apparatus). Such a braking force control type driving force controlling means as described above is superior in a responsive characteristic of the driving force control. However, whenever the slippage occurs on the driven road wheels, the wheel cylinder(s) is activated so that such a frictional body as a brake pad or braking shoe is tended to be worn out.
On the other hand, one of the driving force controlling means includes the driving force controlling means arranged for directly controlling an output of the engine mounted in the vehicle, such as controlling an opening angle of an engine throttle valve disposed within an intake air system of the engine (hereinafter, referred also to as a throttle valve opening angle control type driving force controlling means). Such a throttle valve opening angle type driving force controlling means as described above can achieve a smoother driving force control characteristic through the opening angle control of the throttle valve. However, although the throttle valve is rapidly controlled toward a close direction, a rotation inertia that the engine (or whole drive train) has causes an engine revolution speed not to rapidly be reduced. Hence, the braking force caused by an engine braking given from the engine to the driven road wheels, so-called, a back torque has a possibility of not rapidly reducing the slippage on the driven road wheels. Hence, a sufficient driving force control responsive characteristic cannot be achieved.
Hence, the driving force controlling means arranged for forcefully cutting off or reducing a supply of fuel to the engine in which a combustion state of each combustion chamber is electronically controlled so that the back torque described above can largely and speedily be achieved has been proposed.
In such a fuel cutoff type driving force controlling means as described above, the engine has a plurality of cylinders and is electronically controlled so that a fuel injection for each of the cylinders can individually and independently be controlled. In this case, a slip rate of a driven road wheel velocity (speed) with respect to a target driven road wheel velocity is derived, the number of the cylinders to be fuel supply cutoff is set according to the slip rate, i.e., a desired reduction quantity of the driving force (torque), and the fuel supply is cut off for the set numbers of the cylinders (hereinafter, also referred to as a fuel cutoff) so that the large and speedily back torque causes the suppression or elimination of the slippage on the driven road wheels.
It is noted that, in the above-described type driving force controlling means, the target driven road wheel velocity when the vehicle runs steadily at a vehicle speed higher than a certain vehicle speed is set to a non-driven road wheel speed (hereinafter, simply referred to as a non-driven road wheel velocity). In a case where the vehicle starts, in other words, where the vehicle speed is equal to or below a certain predetermined vehicle speed value, the target driven wheel velocity is set to a predetermined value previously set.
Suppose that both of, for example, the throttle valve opening angle control type driving force controlling means and the fuel supply cutoff type driving force controlling means are installed in the vehicle together in order to compensate for the mutual inconveniences. Since the throttle valve opening angle controlling means is inferior in the response characteristic of the driving force control, the fuel supply cutoff type driving force control, the fuel supply cutoff type driving force controlling means is efficiently acted upon during a start of control to reduce the driving force but thereafter the throttle valve opening angle control type driving force controlling means effectively achieves a smooth driving force characteristic.
Hence, for example, the fuel supply cutoff type driving force controlling means, the number of cylinders for the fuel supply to be cut off are, for example, set in the fuel supply cutoff type driving force controlling means from a proportional component to the slip rate and a differential component having a phase advance with respect to the proportional component. In the other throttle valve opening angle control type driving force controlling means, when a close variable of the throttle valve is, for example, set from a proportional component to the slip rate and an integral component having a phase lag with respect to the proportional component, for example, in order to effectively activate the respective characteristics of both control type driving force controlling means, the differential component of the former may be enlarged, i.e., a differential gain may be enlarged and the integral component, i. e., an integral gain may be enlarged.
In details, in a case where the vehicle runs on a low .mu. road surface having a low frictional coefficient state (simply referred to as a low frictional coefficient state (simply referred to as a low .mu.) between the road surface and each road wheel such as a normal snow road or a steady-state slip occurs on the driven road wheels during a relatively smooth start and acceleration. If even in this state of the steady state slip occurrence, the fuel supply cutoff is carried out, the driving force to the driven road wheels becomes too small so that a sufficient acceleration cannot be achieved. Therefore, in the fuel supply cutoff type driving force controlling means according to the proportional-integral control form, for example, the proportional gain is to a certain degree reduced and if the slip on the driven wheels is converged to some degree, the fuel supply is not cut off or is carried out by a slight quantity and a minute adjustment of the slip rate thereafter is rested on the throttle valve opening angle control type driving force controlling means.
On the other hand, when, in the throttle valve opening angle control type driving force controlling means, both of the throttle valve whose opening angle is adjusted according to a depression of the accelerator pedal and a throttle valve whose opening angle is controlled by means of the throttle valve opening angle control type driving force controlling means are used simultaneously and both are interlocked with each other via the throttle valve, the accelerator pedal is independently moved with no intention (depressed or released) against an intention of the vehicular driver when the driving force controlling means causes the throttle valve to be opened or closed. Or in a case where both accelerator pedal and the throttle valve opening angle control type driving force controlling means are not: interlocked, it is difficult to make the opening angle of the throttle valve which has been controlled by means of the driving force controlling means coincident with the opening angle which is in accordance with a depression angle (rate) of the accelerator pedal.
Therefore, a second throttle valve whose opening angle is adjustable by means of an actuator such as a DC motor or so on (hereinafter, also referred to as a sub throttle valve) is serially and independently installed in the throttle chamber of the engine intake air system with a first throttle valve whose opening angle is adjusted by means of the accelerator pedal (hereinafter, also referred to as a main throttle valve) so that the opening angle of the sub throttle valve is controlled by means of the throttle valve opening angle control type driving force controlling means.
On the other hand, in a case where, in so-called an engine cold state (wherein a temperature of the whole engine is not raised up to a steady state and, hence, a temperature of an engine coolant is not sufficiently raised), a viscosity of an engine lubricant in the engine, i.e., the engine oil is high, a frictional loss (friction loss) of an engine revolution force is large due to the high viscosity resistance. Hence, if is necessary for the engine not to be stalled in order to increase an engine idling speed for the engine to be stopped. In a vehicle in which the combustion state of the engine is electronically controlled, the engine coolant temperature is detected, the opening angle of an idling valve or a combustion state of the engine is managed or adjusted according to -the detected engine coolant temperature so that a fuel rate or percentage in a air-fuel mixture ratio is enlarged to secure a required idling speed.
Hence, as described above, in a case where the main throttle valve and the sub throttle valve are installed together in series with each other, it is necessary to install a mechanical full close stopper on the sub throttle valve so as to provide a clearance through which the intake air flows in the intake air system in order to secure the required intake air quantity to maintain the idling speed at the engine cold state even if the sub throttle valve is at least fully closed.
It is herein defined that the opening angle of the sub throttle valve which corresponds to the clearance through which the intake air quantity passes includes the actual opening angle of the sub throttle valve.
Then, when both means play different roles such that the fuel supply is cut off by the fuel supply cut off type driving force controlling means only when the driving force control is started and thereafter opening angle control of the sub throttle valve is carried out by the throttle valve opening angle control type driving force opening angle control type driving force controlling means, a large slip occurs immediately on the driven road wheels when the vehicle starts on an extremely low .mu. road surface on which the frictional coefficient between the driven road wheels and road surface is extremely low such as a frozen road. To cope with the large slippage, even if the fuel supply cutoff control type driving force controlling means carried out the fuel supply cutoff, the slippage on the driven road wheels is not sufficiently decreased. In this case, the throttle valve opening angle control type driving force controlling means would output a full close command to the throttle valve (especially, the sub throttle valve).
However, the fuel supply cutoff type driving force controlling means which carries out the fuel supply cutoff to the engine only during the increase in the slippage with the differential gain increased as described above does not carry out the fuel supply cutoff or reduction by the slight quantity even if although the slippage on the driven road wheels is not increased any more, the slippage of the driven road wheels itself is not converged into a predetermined state. In addition, since the intake air is sucked into the engine even if the sub throttle valve is fully closed (actually, although the full close command is outputted, the sub throttle valve is not fully closed), a steady state deviation occurs between the engine output under a virtual sub throttle valve fully closed state used in a calculation processing and the actual engine output, It is natural that the actual engine output is larger.
Hence, the steady state deviation of the engine output, i.e., the driving force corresponding to the steady state deviation of the engine output developed between the target opening angle of the sub throttle valve and actual opening angle thereof is continued to be applied to the driven road wheels. In addition, in a case where the road surface on which the vehicle is to start is the extremely low .mu. road surface, the slip is further tended to be difficult to be converged.
This problem may be applied equally well to the case wherein in the driving force controlling apparatus in which both of the throttle valve opening angle control type driving force controlling means and braking force control type driving force controlling means are used simultaneously, the braking force control type driving force controlling means applies to the braking force to the driven road wheels only during an earlier stage of the vehicle start.